Modeling the film casting process using a continuum model for crystallization in polymers

Abstract

In this paper, the film casting process has been simulated using a new model developed recently using the framework of multiple natural configurations to study crystallization in polymers (see Rao and Rajagopal Z. Angew. Math. Phys. 53 (2002) 265; Polym. Eng. Sci. 44(1) (2004) 123; Simulation of the film blowing process for semicrystalline polymers, in press, 2004). In the film casting process, the material starts out as a viscoelastic melt and undergoes deformation and cooling, resulting in a semi-crystalline solid. In order to model the complex changes taking place in the material and predict the behavior of the final solid it is important to use models that are capable of describing these changes. The model used here has been formulated within a general thermodynamic framework that is capable of describing dissipative processes. In addition it handles in a direct manner the change of symmetry in the material; it thus provides a good basis for studying the crystallization process in polymers. The polymer melt is modeled as a rate type viscoelastic fluid and the crystalline solid polymer is modeled as an anisotropic elastic solid. The initiation criterion, marking the onset of crystallization and equations governing the crystallization kinetics arise naturally in this setting in terms of the appropriate thermodynamic functions. The mixture region, wherein the material transitions from a melt to a semi-crystalline solid, is modeled as a mixture of a viscoelastic fluid and an elastic solid. This is in marked contrast to earlier approaches where in the simulation has been done assuming that the material was a viscous fluid and the transition to a solid like behavior is achieved by increasing the viscosity to a large value resulting in a highly viscous fluid and not an elastic solid. The results of our simulations compare well against experimental data available in literature. In addition to these quantitative comparisons have carried out parametric study to study the influence of the different parameters on the film casting process.